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| United States Patent |
5,008,898
|
|
Hamatsu
, et al.
|
April 16, 1991
|
Carrier modulating device for a spread spectrum communication device
Abstract
In a carrier signal modulating device a carrier signal is modulated by
using a square phase sequence as a PN code.
| Inventors:
|
Hamatsu; Mashiro (Tokyo, JP);
Endo; Mamoru (Tokyo, JP)
|
| Assignee:
|
Clarion Co., Ltd. (Tokyo, JP)
|
| Appl. No.:
|
496354 |
| Filed:
|
March 20, 1990 |
Foreign Application Priority Data
| Current U.S. Class: |
375/151; 380/34 |
| Intern'l Class: |
H04L 009/00 |
| Field of Search: |
380/34
375/1
|
References Cited
U.S. Patent Documents
| 4672629 | Jun., 1987 | Beier | 375/1.
|
| 4754465 | Jun., 1988 | Trimble | 375/1.
|
| 4761795 | Aug., 1988 | Beier | 375/1.
|
| 4866734 | Jul., 1989 | Akazawa et al. | 375/1.
|
| 4888787 | Dec., 1989 | Kisak | 375/1.
|
| 4908837 | Mar., 1990 | Mori et al. | 375/1.
|
| 4924188 | May., 1990 | Akazawa et al. | 375/1.
|
| Foreign Patent Documents |
| 63-73730 | Apr., 1988 | JP.
| |
Other References
English-language Abstract of Japanese Reference No. 63-73730 (1 page).
Extracted translation of Japanese Publication No. JP-A-63-73730 (1 page).
|
Primary Examiner: Cangialosi; Salvatore
Attorney, Agent or Firm: Flynn, Thiel, Boutell & Tanis
Claims
What is claimed is:
1. A spread spectrum modulating device comprising:
means for generating first and second carrier signals, which are orthogonal
to each other;
first multiplying means for multiplying a first analog signal corresponding
to a real component of a square phase sequence by said first carrier
signal;
second multiplying means for multiplying a second analog signal
corresponding to an imaginary component of the square phase sequence by
said second carrier signal; and
means for adding the outputs of said first and said second multiplying
means to each other.
2. A spread spectrum modulating device for reference signal for a spread
spectrum communication device using a surface acoustic wave convolver as a
correlator and a square phase sequence as a PN code, comprising:
means for generating first and second carrier signals, which are orthogonal
to each other;
first multiplying means for multiplying a first analog signal corresponding
to a real component of a square phase sequence by said first carrier
signal;
second multiplying means for multiplying a second analog signal
corresponding to an imaginary component of the square phase sequence by
said second carrier signal; and
means for adding the outputs of said first and said second multiplying
means to each other.
3. A spread spectrum modulating device according to claim 1, further
comprising:
memory means for storing phase patterns corresponding to the real and the
imaginary component of the square phase sequence;
a counter for reading out digital data corresponding to each of the phase
patterns from said memory means; and
D/A converting means for converting the digital data thus read out from
said memory means into said first and said second analog signals.
4. A spread spectrum modulating device for reference signal according to
claim 2, further comprising:
memory means for storing phase patterns corresponding to the real and the
imaginary component of the square phase sequence;
a counter for reading out digital data corresponding to each of the phase
patterns from said memory means; and
D/A converting means for converting the digital data thus read out from
said memory means into said first and said second analog signals.
5. A spread spectrum modulating device for reference signal according to
claim 4, wherein said memory means includes a first memory storing a phase
pattern corresponding to the imaginary component of the square phase
sequence in a received signal and a second memory storing a phase pattern
corresponding to the real component of the square phase sequence in the
received signal.
6. A spread spectrum modulating device according to claim 1, wherein the
square phase sequence has elements C.sub.mi which are defined as
C.sub.mi =exp ((2.pi..sqroot.-1/N)mi.sup.2
where N is the code length and is an odd prime number, m=1, 2, . . . , N-1,
and i=0, 1, . . . , N-1.
7. A spread spectrum modulating device according to claim 2, wherein the
square phase sequence has elements C.sub.mi which are defined as
C.sub.mi =exp((2.pi..sqroot.-1/N)mi.sup.2)
where N is the code length and is an odd prime number, m=1, 2, . . . , N-1,
and i=0, 1, . . . , N-1.
Description
FIELD OF THE INVENTION
The present invention relates to a spread spectrum communication device and
in particular to a new carrier modulating device for a spread spectrum
communication device using a square phase sequence as a pseudo noise code.
BACKGROUND OF THE INVENTION
Heretofore various sorts of communication systems have been investigated
and developed. Among them the spread spectrum communication (hereinbelow
abbreviated to SSC) system is well known.
By this SSC system, on the transmitter side, a signal such as data, sound,
etc. having a wide band to be transmitted by using a pseudo-noise code (PN
code) and, on the receiver side, this wide band signal is spread inversely
into the original narrow band by means of a correlator to reproduce the
signal. Recently attention is paid to this communication system, because
it has always a very high reliability from the point of view that it is
strong against external interference and noise, that it has a high
secrecy, etc.
At present, for the wireless SSC, a correlator, which is thought to be the
most simple and convenient and to have a high reliability, is a device
using surface acoustic wave (hereinbelow abbreviated to SAW).
In the SAW correlator there are, in general, those of correlator type
(tapped delay line type) and those of convolver type. Here, although those
of correlator type have a simple construction and generally a high
efficiency, the temperature coefficient of the substrate has remarkable
influences thereon. On the other hand, although those of convolver type
are hardly influenced by variations in the temperature, they have, in
general, a low efficiency. In addition, concerning the PN code described
above, the code is fixed for those of correlator type, while it can be
freely changed for those of convolver type.
Consequently correlators of convolver type are more easily used, provided
that the efficiency is at a practically usable level.
On the other hand, as the PN code used in the SSC system, heretofore binary
sequences such as an M sequence, a GOLD sequence, etc. have been
principally used owing to the simplicity of the code generation. However,
since the cross-correlation value of these binary sequences is not always
small, in the spread spectrum multiple access (hereinbelow abbreviated to
SSMA) communication they cause often cross-talk. As a PN code for the
purpose of solving such a problem, recently a square phase sequence
(minimum cross-correlation multiple phase orthogonal sequence) as
described in JP-A-63-73730 has been proposed.
The square phase sequence is a complex number sequence having a period N (N
being a code length), by which the self correlation function is zero
except for the shifts, which are integer times as long as the code length
N and further the absolute value of cross-correlation functions between
different sequences having a same code length is 1/.sqroot.N, when it is
nomalized, taking the 0-shift component of the self correlation function
as 1. That is, it realizes the mathematical lower limit of the peak value
of the absolute value of the cross-correlation function between orthogonal
sequences.
Although the square phase sequence is a PN code suitable for the SSMA
communication, heretofore no spread spectrum modulating device using the
square phase sequence is known.
Further, as a correlator for the square phase sequence, that described
similarly in JP-A-63-73730 is known. However no case where an SAW
convolver is used as a correlator is studied.
OBJECT OF THE INVENTION
Therefore a first object of the present invention is to provide a spread
spectrum modulating device in the case where the square phase sequence is
used as a PN code.
Furthermore a second object of the present invention is to provide a spread
spectrum modulating device for the reference signal in a spread spectrum
communication device using an SAW convolver as a correlator and the square
phase sequence as a PN code.
SUMMARY OF THE INVENTION
In order to achieve the first object described above, a carrier modulating
device for an SSC device according to the first present invention is
characterized in that it consists of means for modulating a carrier signal
by using a square phase sequence, which comprises a memory for storing
phase patterns corresponding to the real and the imaginary component of
the square phase sequence; a counter for reading out data corresponding to
each of the phase patterns from the memory described above; D/A converting
means for converting the two sets of digital data thus read out from the
memory described above into respective analogue signals; means for
generating carrier signals, which are orthogonal to each other; first
multiplying means for multiplying one of the analogue data signals stated
above by one of the carrier signals stated above; second multiplying means
for multiplying the other analogue data signal by the other carrier
signal; and means for adding the outputs of the two multiplying means to
each other.
Further, in order to achieve the second object described above, a spread
spectrum modulating device for reference signal for an SSC device using a
SAW convolver as the correlator and a square phase sequence as the PN code
according to the second present invention is characterized in that it is
provided with a first memory for storing a phase pattern corresponding to
the imaginary component of the square phase sequence in a received signal;
a second memory for storing a phase pattern corresponding to the real
component of the square phase sequence in the received signal; a counter
for reading out digital data corresponding to each of the phase patterns
from the memories described above; first D/A converting means for
converting the digital data thus read out from the first memory into an
analogue signal; second D/A converting means for converting the digital
data thus read out from the second memory into another analogue signal;
first carrier generating means; second carrier generating means for
generating a carrier signal, whose phase is retarded by 90.degree. with
respect to that of the first carrier signal; first multiplying means for
multiplying the output of the first D/A converting means stated above by
the first carrier signal stated above; second multiplying means for
multiplying the output of the second D/A converting means stated above by
the second carrier signal stated above; and means for adding the outputs
of the two multiplying means to each other.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1(A) and 1(B) show the phase transition state of square phase
sequences C.sub.1 and C.sub.1 *;
FIG. 2 is a block diagram indicating a basic structure of the present
invention;
FIG. 3 is a block diagram illustrating an embodiment of the present
invention;
FIG. 4 is a block diagram illustrating an example of the construction of a
correlating demodulating circuit, to which the present invention is
applied; and
FIG. 5 is a block diagram of another basic structure of the present
invention relating to the correlating circuit described above.
DETAILED DESCRIPTION
Now the working principle of the device according to the present invention
described previously will be explained. An algorithm for generating the
square phase sequence used for realizing the present invention is as
follows. There exist N-1 sorts of square phase sequences having a code
length N (n being an odd prime number). C.sub.m representing them, the
elements thereof C.sub.mi can be expressed, in general, by;
##EQU1##
(Example) Square phase sequences C.sub.1 and C.sub.1 * having a code
length 7 (N=7), which are imaginary conjugates with respect to each other,
are given by;
##EQU2##
A signal B(t) obtained by spread-spectrum modulating (hereinbelow
abbreviated to SS modulating) a carrier signal A(t) with a PN code, which
is a square phase sequence, can be expressed, in general, as follows;
A(t)=.alpha..multidot.cos .omega..sub.c t (4)
B(t)=.alpha..multidot.cos {.omega..sub.c t+.theta.(t)} (5)
where .alpha. represents a proportionality constant representing the
amplitude of the carrier signal; .omega..sub.c the angular frequency of
the carrier signal; and .theta.(t) a phase term variable in time
corresponding to the square phase sequence.
For example, various values of .theta.(t) for different times T
corresponding to the square phase sequences C.sub.1 and C.sub.1 *
expressed by Equations (2) and (3) are given in TABLE 1.
TABLE 1
__________________________________________________________________________
Time
Sequence
.circle.1
.circle.2
.circle.3
.circle.4
.circle.5
.circle.6
.circle.7
__________________________________________________________________________
C.sub.1
0
##STR1##
##STR2##
##STR3##
##STR4##
##STR5##
##STR6##
C.sub.1 *
0
##STR7##
##STR8##
##STR9##
##STR10##
##STR11##
##STR12##
__________________________________________________________________________
FIGS. 1(A) and 1(B) show the phase transition state of the square phase
sequences C.sub.1 and C.sub.1 * given by
Transforming Equation (5), Equation (6) is obtained, as follows;
B(t)=.alpha..multidot.{cos .omega..sub.c t.multidot.cos .theta.(t)+cos
(.omega..sub.c t+.pi./2).multidot.sin .theta.(t)} (6)
FIG. 2 is a block diagram indicating a fundamental circuit for realizing
Equation (6), in which reference numerals 1 and 2 are mixers; 3 is a
carrier signal generator; 4 is a 90.degree. phase shifter; and 5 is an
adder.
That is, in FIG. 2, the output of the carrier signal generator 3 is given
to the phase shifter 4 to generate carrier signals A(t), A'(t), which are
orthogonal to each other and which are applied to the mixers 1 and 2.
After having been modulated with cos.theta.(t) and sin.theta.(t), they are
added to each other in the adder 5. It is understood that the carrier
signal B(t) SS modulated with the square phase sequence can be obtained in
this was.
Here A'(t)=.alpha..multidot.cos(.omega..sub.c t+.pi./2) (4)
is valid.
An embodiment of the present invention based on the working principle
described above will be explained.
FIG. 3 is a block diagram indicating the embodiment of the
spread-spectrum-modulating device according to the present invention, in
which the reference numerals identical to those indicating in FIG. 2
represent identical or analogous circuits and 6 is a clock signal
generator; 7 is a counter; 8 and 9 are memories; 10 and 11 are D/A
converters.
In FIG. 3, the real component (cos.theta.) and the imaginary component
(sin.theta.) the phase pattern 0 corresponding to the square phase
sequence are stored in the memories 8 and 9, respectively, and read-out
with a period of the code length N, responding to a clock signal from the
clock signal generator 6 by means of the counter 7. The two sets of
digital data of the phase pattern thus read-out are converted into
analogue signals by the D/A converters 10 and 11, respectively. Thereafter
they are multiplied by the carrier signals A(t) and A'(t), which are
orthogonal to each other, in the mixers 1 and 2, respectively. The carrier
signal B(t) SS modulated by the square phase sequence is obtained by
adding the output signals from the mixers 1 and 2 by means of the mixer 5.
In this way, it is possible to construct the spread spectrum modulating
device, in the case where the square phase sequence is used as the PN
code, by means of a very simple circuit.
FIG. 4 shows an example of the construction of a correlating demodulating
circuit, in the case where an SAW convolver is used as the correlator and
a square phase sequence is used as the PN code.
In FIG. 4, when a received signal SS modulated with the square phase
sequence and a reference signal are inputted in the SAW convolver 12, an
operation correlating the two input signals is effected in real time in
the SAW convolver 12. The correlation output obtained as the result is
outputted as the correlation demodulation signal through an amplifier 13
and a high pass filter 14.
Now, in order that the self correlation operation of the two input signals
is effected in the SAW convolver 12 to output a spike-shaped correlation
demodulation signal, it is necessary that the square phase sequence for
the received signal and the square phase sequence for the reference signal
are in a relation of complex conjugate to each other.
For example, when C.sub.1 in Equation (2) stated above is the square phase
sequence for the received signal, the square phase sequence for the
reference signal should be C.sub.1 * in Equation (3).
When B(t) in Equation (5) is the SS modulated signal corresponding to the
received signal, the SS modulated signal B*(t) corresponding to the
reference signal can be expressed, in general, as follows;
B*(t)=.alpha..multidot.cos {.omega..sub.c t-.theta.(t)} (7)
Transforming Equation (7),
##EQU3##
is obtained. A fundamental circuit for realizing Equation (8) is indicated
in FIG. 5.
As clearly seen from this figure, it is almost identical to that indicated
in FIG. 2. Although the carrier signal A*(t) for the reference signal
generated by the carrier signal generator 3' is represented by;
A*(t)=.alpha..multidot.sin .omega..sub.c t (9)
which differs in the phase by .pi./2 from the carrier signal A(t) given by
Equation (4) for the received signal, since the two carrier signals are
used usually asynchronously, this phase difference causes no essential
problem.
Further, from FIG. 5, it is understood that the SS modulated signal B*(t)
for the reference given by Equation (7) or (8) can be easily generated
with the same circuit structure, only if the contents in the memories 8
and 9 are exchanged. That is, in order to obtain B*(t), it is sufficient
that the content in the memory 8 is the imaginary component (sin.theta.)
and the content in the memory 9 is the real component (cos.theta.).
As described above, also in the case where the SAW convolver is used as the
correlator and the square phase sequence is used as the PN code, it is
possible to obtain the correlation demodulation signal in a very simple
manner.
As explained above, according to the present invention, it is possible to
realize a spread spectrum modulating device using a square phase sequence
as a PN code by means of a simple circuit construction and the
contribution thereof is great particularly to the practical utilization of
the SSMA communication system having a high reliability.
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